Shear behaviour and stiffness of naturally cemented sands
The behaviour of natural soils is highly influenced by structural features arising from their geological history and was recognised to lie outside current frameworks which account only for the stress-volume state of the soil. The objective of the research was to compare the shear behaviour and stiffness of two naturally cemented sands: a calcarenite with relatively low densities, weak particles and strong bonding and a silica sandstone with high densities, strong particles and weak bonding. Comparative studies were also undertaken on their corresponding reconstituted soils and on an artificially cemented carbonate sand. Testing was performed in stress path controlled triaxial systems over a wide range of confining pressures. Identification of the yield surface was found to be an essential feature to describe the shear behaviour of the soils examined. The determination of the yield points of such stiff soils required internal measurements of stresses and strains, with an accuracy higher than currently achieved in soil testing. For reliable determinations of stiffness and yielding the uniformity of strains in the samples had to be guaranteed. Therefore several parts of the equipment were redesigned or modified and new sample preparation and setting up procedures were developed. Comparison of results from natural and reconstituted soils showed that bonding increased the stiffness and the stress-strain linearity. For the naturally cemented sands the maximum shear modulus was found to vary with state only when the soils were sheared after isotropic yielding. Undrained loading-unloading probes showed that the linear behaviour was reversible and that plastic strains were accompanied by a progressive deterioration of bonding resulting initially in a reduction of the yield stresses and finally, after sufficient cycling in a decrease of the maximum shear modulus. The combined influence of bond strength and specific volume on yielding could be accounted for when normalizing the yield stresses by an equivalent pressure taken on the state boundary in isotropic compression. This type of normalization also allowed the full state boundary to be identified. The yield surface was found to be the boundary limiting the domain governed by bonding and for the calcarenite occupied a larger portion of the permissible states than for the silica sandstone. States between the yield surface and the state boundary surface were controlled by bond degradation and either particle crushing for the calcarenite or dilation for the silica sandstone. After accounting for differences in states, both naturally cemented sands showed peak strengths which were higher than those for the reconstituted soils. For the calcarenite the peak strengths simply resulted from cohesion, as the peak stress ratios were reached on the yield surface. Only when stress ratios at yielding were lower than the critical state stress ratio the strength was truly frictional and coincident with the critical state. For the silica sandstone, in contrast, the peak strengths were found to be frictional except at the lowest confining pressures. The peak stress ratios were reached at states above the yield surface and were associated with a maximum rate of dilation. The dilatancy and strength of the intact soil were higher than those of the reconstituted soil at comparable states and were interpreted as resulting from differences in fabric and from the delayed volumetric response induced by the presence of bonding at the early stages of shearing. The case of the silica sandstone showed that only when density is a predominant factor in comparison with bonding then different modes of shear behaviour follow the theory of Critical State Soil Mechanics.